Alternative Control of Fluctuation Frequency in the Wake of a Rectangulay Cylinder near the Critical Shape

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Numerical simulation by discrete vortex method was carried out on the alternative control between wholly separated flow and periodically reattached flow, triggered off by suction at a leading edge of a rectangular cylinder near the critical shape in a uniform flow. Experiments were executed after preparing rectangular cylinders with the suction slot at the leading edge connected to a suction device in which suction speed and operating time were controllable, based on the results of the numerical simulation. The main results obtained through the simulation and the experiments are the following.1.In the flow around a cylinder with the critical shape, fluctuation in wholly separated flow appeared in the flow around a cylinder with small side-to-height ratio are superposed on periodically reattached flow appeared in the one with large side-to-height ratio. They appear intermittently in the experiment, while supperposed wave form appears in the simulation.2.In the numerical simulation, the … More flow was transferred to the periodically reattached flow by a single suction which had 0.4-0.7 times speed in the incident flow and 1/30 of fluctuating period in the wholly separated flow for the operating time, and the transferred flow returned to the critical flow after a while. The suction width with a half of the height of cylinder was suitable to the transition. This control was possible in the range of side-height ratio between 3.6 and 4.0 at which the critical shape was realized in the numerical simulation.3.In the experiment, the critical shape was realized at 2.7 of side-height ratio at low Reynolds number and 2.3 at high Reynolds number. The results were similar qualitatively to those of the numerical simulation, but, quantitatively, some differences appeared such as the transition occurred at 0.7-1.0 times of the suction speed higher and for 0.1-0.4 times fluctuating period in the wholly separated flow of operating time longer than the numerical results, and the suitable width for the suction control was below 0.2 times the height.4.We have some problems to be investigated such as the differences above-mentioned and the retarded transition occurred at a few periods after the suction. Less